It was narrated that 'Aishah said:
"O Messenger of Allah (ﷺ), what are the things which are not permissible to withhold?" He said: "Water, salt and fire." She said: "I said: 'O Messenger of Allah (ﷺ), we know what water is, but what about salt and fire?" He said: "O Humaira', whoever gives fire (to another), it is as if he has given in charity all the food that is cooked on that fire. And whoever gives salt, it is as if he has given in charity all that the salt makes good. And whoever gives a Muslim water to drink when water is available, it is as if he freed a slave; and whoever gives a Muslim water to drink when there is no water available, it is as if he brought him back to life."
English reference: Vol. 3, Book 16, Hadith 2474
Arabic reference: Book 16, Hadith 2567